Many copiers and printers have the ability to imprint images on media sheets using either a simplex or duplex process. A simplex process is one wherein an image is applied to only a single side of a media sheet being fed through the apparatus, while a duplex process prints images on both sides of the media sheet. Hereafter, the prior art and the invention will be described in the context of a laser printer. However, it is to be understood that it is equally applicable to any system that employs duplex imprinting processes.
Printers which provide duplex imprinting often require highly complex media sheet movement mechanisms to enable both sides of the sheet to be presented to the image exposure system. Such duplex mechanisms require additional space within the printer's covers, create a delay in the print process, and often add considerable expense to the printer mechanism. One of the more advanced printer mechanisms that overcomes many of these drawbacks is the LaserJet series of printers produced by the Assignee of this patent application. As the duplex printing mechanisms incorporated in the LaserJet series of printers are relevant prior art to this invention, their details will be described in conjunction with FIGS. 1-6 hereof.
FIG. 1 shows a schematic side view of a media sheet which will be used to diagram the duplex processes in both the prior art and the invention. The semicircle indicates the top, front side of a media sheet (this is where a letterhead would normally be located). The box indicates the bottom, backside of the media sheet. This is the side of the paper on which the duplex image is printed. A number in either the semicircle or the box indicates the page number which has been or is being printed on that side of the media sheet. An arrow in the middle of the sheet symbol indicates the direction in which the sheet is moving.
FIG. 2 shows a side schematic view of a laser printer 10 which is capable of simplex print operations. Control of laser printer 10 is achieved by microprocessor 11, which receives print orders and data from a connected computer (not shown). Laser printer 10 is provided with two paper feed trays, a multipurpose tray 12, and a larger capacity feed tray 14. Media sheets in multipurpose (MP) tray 12 are oriented as shown by sheet schematic 16 with the letterhead side up and facing closest to the printer inlet. In feed tray 14, media sheets 18 are oriented letterhead side down and arranged so that the letterhead end is closest to input feed roller 20. Depending upon which paper tray is being utilized (either MP tray 12 or feed tray 14), a media sheet will be grabbed by either MP roller 19 or feed roller 20 and passed through a series of rollers 22 into an imprinting path 24.
In the well known manner, a photoconductive (PC) drum 26 is imaged by a laser/scanning system 28. When a media sheet passes between photoconductor drum 26 and transfer roller 30, the media sheet attains a toned image from PC drum 26. From there the sheet passes along imprinting path 24 into a fuser mechanism 32 where the image is fixed, again in the known manner. When the media sheet exits from fuser mechanism 32, it is directed by a simplex/duplex select bar 34 into an output path 36 where it passes between output rollers 38 into an output tray 40. Since the laser printer shown in FIG. 2 has no duplex capability, simplex/duplex select arm 34 is maintained in the position shown in FIG. 2 at all times.
As can be seen from the schematics of the print sheets shown in FIG. 2, feed tray 14 is utilized to provide media sheets for printing. Each page is printed serially and the pages feed, in seriatim, through imprinting path 24 and output path 36 where they exit onto feed tray 40 in page order number, and in a face-down orientation.
Turning now to FIGS. 3-6, a duplexing mechanism 50 employed with the laser printer of FIG. 2 is illustrated. Duplexing mechanism 50 includes a reversing roller 52 and a slave roller 54 that are, respectively, positioned on either side of a reversing path 56. An imprinting path extension 24' communicates with imprinting path 24 within laser printer 10 and is opened when simplex/duplex select lever is in the position shown in FIG. 3. When in that position, select lever 34 closes off output pathway 36 thereby preventing a media sheet from exiting the copier during the duplex printing portion of a print cycle.
Reversing path 56 extends vertically downward to a lower portion of duplexing mechanism 50 where additional duplex transport rollers 58 capture the media sheet and feed it up through additional feed rollers 60, opening 62 and back into imprinting path 24.
The reversing action of duplexer 50 is controlled by an optical sensor 64 which is positioned immediately below reversing roller 52. Optical sensor 64 detects the end of a media sheet as it is fed vertically upward by reversing roller 52. At that point, a signal from optical sensor 64 causes a reversal of the rotation of reversing roller 52 which sends the media sheet downward along duplex path 56, etc.
In FIG. 4, a timing diagram is illustrated for the prior art duplexing system shown in FIGS. 3 and 5, 6. As will be hereinafter seen, even numbered pages (i.e. backsides) are printed first to preserve proper page order in the output tray. The horizontal axis of the chart of FIG. 4 indicates increments of time and each line of the chart indicates a specified station in the laser printer, and the numbers above each line indicate when the page bearing that number arrives at the station. For instance, the first line of the chart of FIG. 4 indicates that the pages are received serially from a connected computer (not shown). Note that no printing action is commenced at PC drum 26 until both pages 1 and 2 are stored in memory, at which time, page two is printed first. As is shown by dashed line 80, page two then proceeds through reversing roller 52 and passes into the lower portion of duplexer mechanism 50. The sheet bearing page two is then transported past PC drum 26 to enable page one to be printed on its front side. The duplex-printed media sheet bearing pages one and two is then passed to output roller 38 where it exits onto output tray 40 and is released from memory.
The prior art duplex printing operation will now be described in conjunction with FIGS. 3 and 5-7. It is assumed that large capacity feed tray 14 is being used to feed media sheets into the duplex printing mechanism. As can be seen in FIG. 3, page two has already been printed and is being passed upwardly between reversing roller 52 and slave roller 54 in duplexing path 56. It will be noted that the orientations of the media sheets in MP tray 12 and feed tray 14 must be "flipped" by the user in order to allow the duplex printing operation to output the media sheets in page number order.
In FIG. 5, the sheet bearing page two has already passed down through reversing path 56, rollers 58 and 60, and is being fed upwardly through opening 62 into imprinting path 24 to enable page one to be imprinted thereupon. Additionally, page four has already been printed on a second media sheet, which sheet is being directed upwardly between reversing roller 52 and slave roller 54.
In FIG. 6, the next stage of operation is shown wherein simplex/duplex select arm 34 has been pivoted clockwise to open output path 36 to the media sheet bearing pages one and two as it exits from fuser 32. At this stage, the sheet bearing page four resides in the lower portion of duplexer 50 awaiting the printing of a next even page (page 6) on a sheet presently in imprinting pathway 24.
In FIG. 7, the next stage of operation is shown wherein the sheet bearing page six has been allowed to enter duplexing path 56 as a result of switching of simplex/duplex select arm 34. Additionally, the sheet bearing page four has been passed into imprinting path 24 so as to enable page three to be imprinted on its front side. After the sheet bearing page six passes out of fuser 32 and clears simplex/duplex select arm 34, that arm is switched so as to open output path 36, to enable the sheet bearing pages three and four, when it exits from fuser 32, to pass into output path 36. The process continues (as shown in the chart of FIG. 4) until all eight pages have been imprinted and outputted.
As above indicated, in order for the media sheets to be passed to output tray 40 in page order, it is required that the arrangement of sheets that feed laser printer 10 be altered. Often, a user will forget to reorientate the pages and will thereby not achieve the desired letterhead/page numbering arrangement when using the simplex process.
Accordingly, it is an object of this invention to provide a duplexing mechanism for an image imprinter that enables common sheet orientations to be used, whether the imprinting mechanism is employed for simplex or duplex operation.
It is a further object of this invention to provide an improved duplexing mechanism for a laser printer that does not extend print time as a result of its operation.
It is yet another object of this invention to provide an improved duplexing mechanism for a laser printer which employs minimum added apparatus to enable simplex/duplex printing to occur without requiring reorienting of media sheets.